1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a battery pack that enables its outer case to be formed in small thickness regardless of formability and material property and has enhanced twisting resistance.
2. Description of the Related Art
Generally, a battery pack uses a chargeable/dischargeable secondary battery because of its low cost. The secondary batteries include a nickel-cadmium (Ni—Cd) battery, a nickel-hydride (Ni-MH) battery, a lithium (Li) battery, a lithium-ion (Li-ion) battery and a lithium polymer battery.
The battery pack is usually constituted of a chargeable/dischargeable bare cell, a protection circuit board electrically coupled to the bare cell so as to control charging/discharging and interrupt the circuit in a case of over-charge/discharge, a resin filled between the bare cell and protection circuit board so as to prevent the protection board from being separated from the bare cell, and an outer case formed in a shape mountable on an external set while packing the bare cell, protection circuit board and resin.
The battery pack is manufactured by connecting a lead to a positive electrode of a bare cell and connecting a PTC thermistor to a negative electrode, and electrically connecting a protection circuit board to the lead and PTC thermistor. Next, a resin is filled in a gap between the protection circuit board and bare cell so as to mechanically fix the protection circuit board to the bare cell, thereby preventing the protection circuit board from being separated from the bare cell. The integrated bare cell and protection circuit board are packed by an outer case having a shape mountable on an external device. Herein, the outer case is formed integrally with the bare cell, protection circuit board and resin by being molded with other resin.
The battery pack should be small in size and slim in shape for matching the small size and lightweight of electronic devices. Structure of the outer case constituting an appearance of the battery pack is important for making the battery pack slim.
In other words, slim shape of the battery pack can be determined by thickness of each of the bare cell battery, protection circuit board and outer case. The larger a capacity of the bare cell battery is, the thicker battery pack becomes. Thus, the thickness of the outer case should be thin for high capacity and slim shape of the battery pack.
However, in the case of an outer case formed by using an integrated type molding in a contemporary battery pack, there is a limitation to reduce the thickness of the outer case by formability and material property which are technical restrictions in an injection molding method. For example, it is actually difficult to make minimum thickness of the outer case smaller than 0.22 mm in the contemporary battery pack. Further, because the outer case of the contemporary battery pack is integrally formed, it is relatively weak to twisting.